1. Field of the Invention
The present invention relates to the design of a rotor of an electric machine, such as a cryo-turbogenerator, with a superconducting field winding and more specifically, to the design of an assembly for feeding the coolant into the winding cooling zone.
2. Description of the Prior Art
In the rotors of electric machines, particularly cryo-turbogenerators, the material preferably used as a superconductor in the field winding is a niobium-titanium alloy which exhibits, with large magnetic fields and large currents, a critical temperature close to the boiling point of liquid helium here used or a cooling medium, at atmospheric pressure. With temperatures in the superconducting field winding zone exceeding a critical temperature close to the boiling point of liquid helium, a quench takes place.
As the electric machine is operated, the coolant is fed to the cooling zone along the rotor axis through a duct located in, or close to, the central space of the rotor. The location of the superconducting field winding on the outer surface of the rotor support structure leads to the necessity of radial coolant transfer from the axial duct to the winding area. Now, as a result of a centrifugal compression due to adiabatic heating, the temperature of the coolant in the radial duct increases towards the rotor periphery, and at the outlet of the radial duct it exceeds the coolant temperature at its inlet by 0.5.degree.-1.degree. K., thereby causing the temperature margin of the superconducting field winding to be reduced, and in some cases leading to a quench.
The problem of feeding the coolant having a temperature such as to avoid the risk of losing the effect of winding superconductivity can be handled by supplying an overcooled coolant to the radial ducts, its subsequent heating therein being compensated by the overcooling temperature. The extra overcooling of the coolant at temperatures of 3.degree.-4.degree. K., however, makes it necessary to provide additional devices and requires an increased power consumption.
Another way of handling the problem has the advantage of cost effectiveness: it involves removal of the heat released in the coolant within the radial duct.
Known is the rotor of an electric machine with a superconducting field winding, comprising a hollow support structure with the superconducting winding disposed thereon. The internal cavity of the frame has provided therein a trough made of a high thermal conductivity material. The trough communicates, via tubes mounted in the radial holes provided within the large tooth of the support structure, with a ring-type header disposed below the shrouding cylinder. The tubes are made of a high thermal conductivity material, such as copper, having a good thermal contact with the trough and being heat-insulated from the support structure by tubes of a low thermal conductivity material, such as glass-cloth-base laminate U.S. Pat. No. 4,238,701). In the rotor described, the coolant, i.e. liquid helium, is supplied over the helium transfer coupling to the trough, and, then, along the radial tubes and through the ring header, it is fed to the field winding. The heat released within the radial duct due to centrifugal compression of liquid helium is removed through the radial tubes and the trough, being in thermal contact therewith, into the helium vapor pool in the internal cavity of the support structure.
In the course of operation of the electric machine, however, and particularly under transient conditions, an amount of heat is supplied to the ring header through the shrouding cylinder which may produce a helium countercurrent flow in the radial tubes and hence result in an increased helium temperature in the cooling zone and in a quench.
Moreover, the assembly of such rotor presents difficulties resulting from the need of welding the heat-conducting tubes to the trough to obtain a safe thermal contact, the operation being performed with a small radius and great length of the internal cavity of the support structure.